Hitherto, tin oxide particles, antimony-containing tin oxide particles, tin-containing indium oxide particles and the like are known as materials for transparent electrically conductive paints. Among them, tin-containing indium oxide particles comprising indium oxide containing tin is used to coat the surface of a cathode ray tube (CRT) and a liquid crystal display (LCD), which requires antistatic properties and electromagnetic wave shielding properties, since they have good light transmission to visible light and high electrical conductivity. Furthermore, a sheet on the surface of which is coated with such particles is expected to find a wide variety of applications such as a touch panel besides the displays because of its good light transmission and electrical conductivity.
The tin-containing indium oxide particles are transparent to visible light and also semiconductive materials which exhibit electrical conductivity due to oxygen deficiencies. That is, it is envisaged that tin atoms in indium oxide may form Sn4+ and act as electron donors and thus the indium oxide particles have high conductivity.
When the tin-containing indium oxide particles are dispersed in a binder and coated on a substrate, their particle size should be usually a half or less of the wavelength of light to achieve high transparency. Therefore, the particle size should be 200 nm or less so that a coating film containing the particles is transparent to visible light.
To produce such tin-containing indium oxide particles, for example, JP-A-62-7627 discloses a method comprising adding an aqueous alkaline solution such as aqueous ammonia or an aqueous ammonium carbonate solution to an aqueous solution of a mixture of indium chloride and tin chloride to coprecipitate hydroxide of indium and tin, heating the hydroxide to form tin-containing indium oxide and then mechanically milling the oxide to obtain fine particles of tin-containing indium oxide. In the Examples of this JP application, tin-containing indium oxide particles having an average particle size of 0.1 μm was produced by heating and subsequent mechanical milling.
JP-A-5-201731 describes that it is important for the achievement of high conductivity to adjust the content of sodium and potassium to a specific amount or less when the mixed hydroxide of indium and tin, which is formed by coprecipitation in the same manner as JP-A-62-7627, is sintered and milled to obtain tin-containing indium oxide particles. JP-A-5-201731 describes that tin-containing indium oxide particles having a particle size of 0.01 to 0.03 μm were obtained after milling.
JP-A-2001-220137 describes that agglomerates are less formed during sintering at a higher heating temperature, when potassium is present when the coprecipitated hydroxide of indium and tin are sintered to form tin-containing indium oxide particles, and further that tin-containing indium oxide particles having a particle size of 0.12 to 0.3 μm are obtained after sintering and milling.
JP-A-6-232586 discloses an electromagnetic wave-shielding film comprising fine conductive particles, which contain indium oxide and tin oxide, and describes that it is essential for the conductive particles to be granular particles having a particle size of 30 to 50,000 Å in order to maintain the transparency of the film and the dispersibility of the particles. Example 1 of this JP patent application produced tin-containing indium oxide fine particles having a cubic particle shape.
In general, the coating film of tin-containing indium oxide particles is formed by applying a paint, which comprises such particles dispersed in a solvent containing an inorganic or organic binder, on various substrates. In such a case, not only the particles are minute or fine particles but also the particles are uniformly dispersed in the film to achieve the transparency of the coating film.
However, when the particles are made minute or fine, they tend to form secondary agglomerates in the paint and thus it is difficult to obtain a uniform dispersion. If such agglomerates are present in dispersion, the coating film has decreased electrical conductivity and also the transparency of the film deteriorates.
An essential problem of the tin-containing indium oxide particles is that the electrical conductivity of the film decreases as the particle size decreases. A reason for this is as follows: The electrical conductivity of the coating type electrically conductive film is achieved by flowing the electricity through contacts between the particles. In the case of granular particles, since the contacts are point contacts, the number of point contacts increases as the particle size decreases, so that the resistance as a whole increases. That is, although the tin-containing indium oxide particles themselves are electrically conductive, the conductivity of a coating film containing the particles largely depends on the contact resistance between the particles.
Therefore, the transparency and the electrical conductivity of the coating film containing the tin-containing indium oxide particles have a trade-off relationship, since the particle size should be made as small as possible to achieve the transparency of the film, but the number of point contacts increases and thus the electrical resistance increases as the particle size decreases.
The coating film comprising the tin-containing indium oxide particles uses particles having a suitable particle size which can attain a good balance between the transparency and electrical conductivity of the film. However, the properties of such a coating film are inferior to the properties of a film of tin-containing indium oxide which is formed by vapor deposition or sputtering. Thus the coating film comprising the tin-containing indium oxide particles may not sufficiently make use of the coating easiness and low cost of the paint, and can be used in very limited applications.